The present invention relates to an electromagnet, and more particularly to an electromagnet and a method for producing the same wherein the electromagnet has an attractive force generated by magnetic flux due to a d.c. current or rectified a.c. current.
It is well known that electromagnetic contactors are broadly employed for opening and closing operations of various circuits. That is, a movable contact is maintained contacted to a fixed contact connected to a main circuit by passing a current through the electromagnet so that the circuit is closed. On the other hand, the circuit is opened by stopping the current to the electromagnet. An outline of this type of electromagnet contactor will now be described with reference to the accompanying drawings.
In FIGS. 1 and 2, breaks are provided at portions of a current passage 10a, 10b and 10c supplying a threephase a.c. source to the main circuit. Movable-contactors 16 which are pushed and held through a compression spring 18 by a cross bar 14 are movable disposed upwardly and opposed to the fixed-contactors 12. A return spring (not shown) is provided between the cross bar 14 and the fixed-contactor part so as to separate the movable-contactors 16 upwardly from the fixed-contactor 12. Also, the cross bar 14 is lowered against the return coil spring by the electromagnet 20 provided below. The electromagnet 20 is composed of an "I" shaped movable-iron core 24 having a contact surface 22 fixed with degree of freedom to the lower end of the cross bar 14 and an "U" shaped fixed-iron core 28 having a contact surface 26 confronting the contact surface 22. An operational coil 32 is wound around one of magnetic legs 30 of the fixed-iron core 28.
A driving circuit for exciting the operational coil circuit is shown in FIG. 3. An a.c. source is connected through a starting switch 35 and an exchange circuit 36 to a rectifier circuit 38 whose outputs are applied to the operational coil 32. The exchange circuit 36 is composed of a normally-closed switch 40, resistor 42 and a condenser 44 for dividing the voltage. The latter two elements are in parallel with the normally-closed switch 40. Further, the switch 40 is opened when the movable-iron core 24 is attracted to the fixed-iron core 28, i.e., when the main circuit is switched on. In addition, reference numeral 46 designates a varistor for constant voltage. In the formation of a closed loop to the main circuit, the a.c. source 34 is coupled through the switch 40 to the rectifier circuit 38 by closing the starting switch 35, so that the rectified output excites the operational coil 32. Therefore, the movable-iron core 24 is attracted to the fixed-iron core 26, that is, the cross bar 14 descends against the return coil spring so that the movable-contactor 16 is contacted to the fixed-contactor 12. As a result, current flow to the main circuit is initiated.
After the formation of a closed loop to the main circuit is completed, the normally-closed switch 40 is opened so that the a.c. source 34 supplies a smaller exciting current to the operational coil 32 through the resistor 42 and condenser 44 than that at the time of loop closure. That is, the attraction of the movable-iron core 24 toward the fixed-iron core is maintained by a smaller exciting current.
By the way, in the above electromagnet, the moving-iron core 24 and the fixed-iron core 28 are attracted to each other and contacted by switching on a current, and are detached from each other by switching off this current. However, many on-off operations cause the generation of (magnetic) remanence in the core so that it is impossible to detach the moving-iron core 24 from the fixed-iron core 28 by turning off the current. Also, the contacting surfaces of both the moving-iron core and the fixed-iron core suffer a shock when the surfaces come together by turning on the current, resulting in an abrasion and deformation of the iron core. Therefore, it is required to impart abrasion resistance and shock strength to the iron cores.
Various attempts for improving the conventional defects have been proposed (see, for example, Japanese Patent publication No. 20096/65 and Japanese Utility Model publication No. 19269/71).
Also, it has been proposed that a non-magnetic layer having a thickness of 2-3 .mu.m be formed on the contacting surfaces of at least one between the moving-iron core and the fixed-iron core by hard chrome plating or the like under conditions such as those of the plating bath composition, temperature and current density below.
TABLE ______________________________________ Chromic acid anhydride 150 g/l Sulfuric acid 1.8-1.5 g/l Specific gravity 13.5 Temperature 45-55.degree. C. Current density 10-80 A/cm.sup.2 ______________________________________
In this case, it is very difficult to uniformly form a non-magnetic layer having a desired thickness of 0.08-0.2 mm. Furthermore, the magnetic characteristics of the iron cores are unstable and excellent abrasion resistance and shock strength cannot be obtained from the conventional chrome plating layer.